Generally, bottle cans that are filled with drinks and the like are formed in the following manner.
Firstly, as shown in FIG. 7A, a close-ended cylindrical body 1 is formed by performing drawing processing and ironing processing on a metal plate made of aluminum alloy or the like. Next, neck-in processing is performed on the aperture portion of the cylindrical body 1. As a result, as shown in FIG. 7B, a bottle can body 5 is formed that is provided with a large diameter drum portion 2, a shoulder portion 3 that is joined to a top end in the can axial direction of the drum portion 2 and whose diameter gradually narrows as it moves upwards, and a pipe sleeve portion 4 that is joined to a top end of the shoulder portion 3 and extends upwards. In addition, as shown in FIG. 7C, in the pipe sleeve portion 4 of the bottle can body 5, after the diameter of portions excluding a bottom portion 4a has first been widened, diameter-narrowing processing is performed on the wide diameter portion with the position of the processing shifted sequentially upwards in the can axial direction. As a result, the bottle can body 5 shown in FIG. 7D is formed.
The pipe sleeve portion 4 of this bottle can body 5 is provided with a skirt molding pre-form portion 4b that is joined to a top end in the can axial direction of the pipe sleeve portion bottom portion 4a and bulges outwards in the radial direction, a male threaded molding pre-form portion 4c that is joined to a top end of the skirt molding pre-form portion 4b and extends upwards, and a curl molding pre-form portion 4d that is joined to a top end of the male threaded molding pre-form portion 4c and has a narrower diameter than the molding pre-form portion 4c. Note that the top end portion in the can axial direction of the pipe sleeve portion bottom portion 4a forms a neck portion molding pre-form portion 4e that has a narrower diameter than the skirt molding pre-form portion 4b. 
Next, as shown in FIG. 8, a spiral male threaded portion 6 is formed in the male threaded molding pre-form portion 4c of the pipe sleeve portion 4 of the bottle can body 5, an internal surface of the skirt molding pre-form portion 4b is pressed towards the outer side in the radial direction, and thereby a skirt portion 112 has been molded. Thereafter, a curl portion 8 is molded by bending the curl molding pre-form portion 4d outwards in the radial direction. As a result, a bottle can 110 having a pipe sleeve portion 111 is formed. As shown in FIG. 8, the skirt portion 112 has a schematic structure that is provided with a first circumferential wall portion 112a whose diameter becomes gradually wider as it moves downwards in the can axial direction, and with a second circumferential wall portion 112c that is joined to a bottom end of the first circumferential wall portion 112a via a convexly curved surface portion 112b which protrudes towards the outer side in the radial direction, and whose diameter becomes gradually narrower as it moves downwards.
Note that in the skirt portion 112, conventionally, only the molding pre-form portion of the convexly curved surface portion 112b of the internal surface of the skirt molding pre-form portion 4b is pushed towards the outer side in the radial direction, and respective molding pre-form portions of the first circumferential wall portion 112a and the second circumferential wall portion 112c are deformed so as to follow the deformation towards the outer side in the radial direction of this molding pre-form portion, and are then molded into the first circumferential wall portion 112a and the second circumferential wall portion 112c. As a result of this, the skirt portion 112b is molded. Moreover, structures disclosed in Patent Document 1 given below are known for the apparatus and method that are used to mold the male threaded portion 6.    PATENT DOCUMENT 1: Japanese Patent Application, First Publication, No. 2002-219539